This invention relates to novel water absorptive resins which have high water absorptivity and are capable of retaining their shape after absorption of water and of being molded into a variety of forms such as film, fiber and molded materials.
Water absorptive resins are used for many applications as absorbent materials for hygienic media such as paper diapers, sanitary napkins, water retentive agents for agriculture and gardening, and other industrial materials.
Resins currently available as water absorptive resins are crosslinked alkali metal salts of polyacrylic acid, crosslinked products of alkali metal acrylate grafted starch, crosslinked copolymers of vinyl alcohol and alkali salts of acrylic acid, crosslinked copolymers of vinyl alcohol-maleic anhydride, and the like. Each of these resins has a crosslinking structure for sustaining its shape after absorption of water.
The use of the water absorptive resins has recently become widespread, and furthermore the water absorptive resins are increasingly required to have shapes in use of not only powder but also film, fiber and molded articles.
However, the currently available water absorptive resins are crosslinked products and thus are not thermoplastic, so that it is difficult to form them into film or fiber. Furthermore, even if films can be obtained by compression molding, such products may be disadvantageous because of their low impact strength.
There have been many researches on water absorptive resins which are capable of being molded into film or fiber in order to solve the above problem. But most of their results have been unsatisfactorily. For instance, the method wherein a thermoplastic resin of excellent moldability such as polyethylene, polypropylene or the like is blended with a carboxylate-containing water absorptive resin afforded no uniform dispersions because of poor compatibility of the thermoplastic resin with the water absorptive resin. Accordingly, researches were conducted on the blending of a water absorptive resin with a polar group-containing thermoplastic resin, which is recognized to have a good affinity with the water absorptive resin. Polar group-containing thermoplastic resins such as an ethylene-vinyl acetate copolymer (EVA), an ethylene-ethyl acrylate copolymer (EEA) and an ethylene-acrylic acid copolymer (EAA) exhibited excellent compatibility with carboxylate-containing water absorptive resins and produced uniform dispersions, but the dispersions were inferior in water absorption property and poor in shape maintenance after absorption of water. They also have the problem of peeling off of the water absorptive resin.